Will the foundations of my extension be OK to take another storey?
Generally the design for foundations of a single storey building is the same as that for a two storey extension. The important aspects include:
depth to the bottom of the foundations below ground level,
depth of concrete in the foundations,
width of the foundation concrete,
location of the wall within the width of the foundation and
probably even more important, the nature of the ground under the foundation.
Unless the original foundations were laid within the last 5 years it will be necessary to expose the existing foundation in one or two locations so that the above aspects can be assessed by a structural engineer or the Building Control Surveyor before planning a first floor extension on an existing single storey part of the building.
Other important aspects to consider are:
suitability of existing roof structure to act as a floor
suitability of existing lintels over ground floor openings